Answer:
The average temperature of the atmosphere outside the airplane is [tex]206.25\,K[/tex].
Step-by-step explanation:
The average temperature of the atmosphere outside an airplane flying at an altitude of 12600 meters is computed by evaluating the linear function:
[tex]T (12.6\,km) = 288.15 - 6.5\cdot (12.6\,km)[/tex]
[tex]T (12.6\,km) = 206.25\,K[/tex]
The average temperature of the atmosphere outside the airplane is [tex]206.25\,K[/tex].